Glow discharge method and apparatus



June 18, 1963 R. L. JEPSEN 3,

GLOW DISCHARGE METHOD AND APPARATUS Filed 00L 6, 1960 l 2 Sheets-Sheet 1INVENTOR.

P0527? Z.Jpse7z usable pumping speed.

United States P lm Or 3 094 639 GLOW DISCHARGE METHon AND APPARATUSRobert L. Jepsen, Los Altos, Calili, assignor to Varian tssociates, PaloAlto, Calili, a corporation of Caliornia Filed Oct. 6, 1960, Ser. No.60,819 Claims. (Cl. 313-7) The present invention relates in general toglow discharge getter ion vacuum pump apparatus and more particularly toa method of improving the pumping speed of such devices. Such a vacuumpump is extremely useful for providing uncontaminated high vacuum asrequired in many devices.

Heretofore, vacuum pumps have been built having for their principle ofoperation the establishment of a glow discharge within the interior ofeach of a plurality of open-ended tubular anode cells disposed betweenand spaced apart from two cathode plates and having a magnetic fieldthreaded through the anode cells. Positive ions produced by the glowdischarge are directed against the cathode plate. In the pump, theimpinging ions produce sputtering of a reactive cathode material. Thesputtered material is collected upon the interior surfaces of the pumpwhere it serves to entrap molecules in the gaseous state coming incontact therewith. Some atoms are buried in the cathode plates. In thismanner, the gas pressure within the vessel enclosing the cathode andanode elements is reduced.

The pumping speed per unit of cathode area increases with increasingmagnetic field intensity B assuming the cells can be aligned in thefield B. In the interest of reducing leakage flux the gap width betweencathode plates is preferably relatively short compared with thetransverse dimension of the gap. For a given magnetic field intensity Band gap width the problem is to utilize the field B as eiiiciently aspossible.

The intrinsic pumping speed of such pumps increases substantiallylinearly with the height of the anode cells, but as the height of theanode cells approaches the width of the gap between the cathode platesthe usable pumping speed approaches zero because of conductancelimitations.

According to the teachings of the present invention, once given the sizeof the magnet air gap in a getter ion vacuum pump the optimum spacingbetween the cathode and anode can be determined to obtain themaximum Theprincipal object of the present invention is to pro vide a novelimproved glow discharge getter ion device with the proper electrodespacings for a given gap between cathode plates whereby the desiredoperating performance of the pump which is dependent upon gas access tothe pumping assembly can be obtained.

One feature of the present invention is the provision of a method forselecting the cathode to anode spacing in a glow discharge getter ionpump so that the usable pumping speed of the vacuum pump is maximized.

Another feature of the present invention is the provision of a methodfor selecting the anode height in a glow discharge getter ion pump for agiven gap between cathode plates so that the desired operatingperformance of the pump can be obtained.

Another feature of the present invention is the provision of a novelglow discharge getter ion pump with the cathode spacing in the range of1.15 to 1.35 inches and with the cathode to anode spacing in the range.20

to .35 inch. I 7

Other features and advantages of the present invention will becomeapparent upon a perusal of the specification taken in connection withthe accompanying drawings, wherein:

FIG. 1 is a schematic block diagram depicting a typical evacuationsystem utilizing the novel vacuum pump of the present invention,

FIG. 2 is a side view partly in cross section of a novel electricalvacuum pump apparatus of the present invention,

FIG. 3 is a top view partly in cross section of a novel electricalvacuum pump apparatus of the present invention,

FIG. 4 is a cross sectional view of a portion of the structure of FIG. 2taken along line 4-4 in the direction of the arrows,

FIG. 5 is a graph showing the pumping speed per unit of anode length vs.the cathode-anode spacing,

FIG. 6 is a graph showing the conductance into the anode cellularcompartments as a function of the cath ode-anode spacing, and

FIG. 7 is a graph of usable pumping speed per unit of anode length vs.the cathode anode spacing for a given gap between cathode plates.

Referring now to FIG. 1 there is shown in schematic block diagram formthe novel electrical vacuum pump of the present invention as utilizedfor evacuating a given structure. More specifically, an electricalvacuum pump 1 is connected via a hollow conduit 2 to a compression port3 and thence via a hollow conduit 4 to a structure 5 which it is desiredto evacuate. The compression port 3 serves to provide a coupling wherebythe structure '5 and associated conduit 4 may be removed and replaced byanother structure and conduit for successive evacuation of a pluralityof structures 5. A mechanical vane pump 6 is also connected to thecompression port 3 via conduit 7 and pinch-oft valve '8. To evacuate thestructure 5, the mechanical vane pump is put into operation serving toreduce the pressure within the structure 5 to between 5 and 20 or lowermicrons at which point the valve 8 is closed and the electrical vacuumpump 1 started. Pump 1 is supplied'with operating potentials firom asource 9 as, for example, a 60-cycle power line via transformer 11. Thesecondary of transformer 11 is pro- .vided with a rectifier 12 and ashunting smoothing capacitor 13 whereby a DC. potential may be appliedbetween anode and cathode members of the electrical vacuum pump 1, whichwill be more completely described below. Although a preferred embodimentutilizes a DC. potential, A.C. potentials are also operable.

Referring now to FIGS. 2, 3 and 4, the electrical Vacuum pump 1 includesa vacuum tight envelope 14 as of, for example, stainless steel. Theenvelope 14 is provided with a central rectangular chamber 15 having apair of outwardly extending lesser rectangular chambers 16 communicatingwith the central rectangular chamber bottom end walls 17 and 18respectively, suitably sealed to the side walls of the envelope 14 as byheliarc welding. A cylindrical exhaust tubing 19 such as stainlesssteel, is fixedly secured in a vacuum tight manner to the top wall 17surrounding an aperture therein and thereby communicates with thecentral chamber 15. An annular flange 21 is carried from the .top end ofthe exhaust tubing 19 for connecting the exhaust tubing 19 and hence theelectrical vacuum pump 1 in a vacuum tight manner to the hollow conduit2. In practice the conduits 2, 4, and 7 will be as wide as the exhausttubing 19 but are shown smaller inwthe drawing for sake of convenience.

The pumping assemblies of the electrical vacuum pump 1 are carriedwithin the two lesser rectangular chambers 16. The pumping elementsinclude two mutually parallel spaced apart cathode plates 22 of areactive material such as titanium and a rectangular cellular anode asof, for example, titanium 23. The cellular anode is carried between andspaced from the cathode plates 22 by means of a plurality of insulatorspacers as of alumina ceramic. The anode 23 is held on the spacers 24 bymeans of a pair of snap rings 25 which snap into annular grooves in theoutside surface of the spacers 24. The spacers 24 serve to insulate theanode from the cathode and present the correct spacing between thecathode plates 22 and the cellular anode 23. A hollow cylindricalsputter shield 26 is provided around each end of each spacer 24 toshield the insulator spacer 24 from sputtered cathode material whichmight otherwise coat the insulator spacers 24 and produce unwantedvoltage breakdown or current leakage thereacross.

The pumping assembly is secured within one of the lesser rectangularchambers 16 by means of bolts 27 which pass through apertures in aportion of the cathode plates 22 which project from the main part of thepumping assembly and are curved to lie against a wall of the centralrectangular chamber 15 when the remainder of the pumping assembly ispositioned within one of the lesser chambers 16.

High voltage is supplied to the anode 23 within the electrical vacuumpump via the intermediary of a high voltage lead-through insulatorassembly 28 including a conductive rod 29 adapted for connection to thepower source 9 and vacuum sealed within a hollow cylindrical insulator31 such as alumina ceramic. The hollow insulator 31 is connected to thevacuum envelope 14 in a vacuum tight manner by an annular insulatorframe 32 such as Kovar. Within the vacuum tight envelope 14 a lead 33connects the conductive rod 29 to the cellular anode 23.

A magnetic field, typically between 1,000 and 2,000 gauss, is appliedperpendicularly to the cathode plates 22 by a plurality of: rectangularmagnets 34 such as fenrite magnets. The magnets 34 are fixedly securedto rectangular pole pieces 35, which are fixedly secured to the pumpenvelope 14 by any desired means. A pair of handles 36 are fixedlysecured to each of the pole pieces 35 for easy handling of the pump.

In operation, a positive potential, typically between 3 and kv., issupplied to the anode 23 via the conduc tive rod 29 and the lead 33. Thevacuum envelope 14 and therefore the cathode plates 22 are preferablyoperated at ground potential to reduce hazard to operating personnel.With these potentials applied a region of intense electric field isproduced between the cellular anode 23 and the cathode plates 22. Thiselectric field produces a breakdown of the gas within the pump resultingin a glow discharge within the cellular anode 23 and between the anode2-3 and cathode plates 22. The glow discharge results in positive ionsbeing driven into the cathode plates '22 to produce dislodgment ofreactive cathode material which is thereby sputtered onto the nearbyanode 23 to produce gettering of molecules in the gaseous state comingin contact therewith. Other atoms bury themselves in the cathode plates.In this manner, molecules from the entire system flow into the regionbetween the cathode plates 22 and are pumped, and the pressure withinthe vacuum envelope 14 and therefore structure's communicating therewithis reduced.

Economics and other considerations as stated above will determine themagnet air gap. The width of the magnet air gap will essentiallydetermine the width g of the gap between the cathode plates 22 becauseboth the thickness of the envelope wall 14 and the thickness of thecathode plates are relatively standard in pumps with pumping speeds inthe range of 100-5,000 l./sec., and which pump down to pressures on theorder of 1 l0 of Hg. Typically, the envelope 14 will be 100 mils thickand the cathode plates 90-120 mils thick.

The intrinsic pumping speed S of an entire pumping assembly made up of Nanode cellular compartments 1S S=NS (1) where S is the intrinsic speedof a single cellular oompart-rnent. The intrinsic speed S variessubstantially linearly with the height h of the cellular compartment.Therefore, the following relation exists where K is a function of thevoltage V, the magnetic field intensity B, the cell geometry, thecathode material, the gas being pumped and its pressure and a is thecathodeanode spacing. Therefore, Equation 1 can be rewritten as follows:

The total number of cellular compartments N can be expressed as theproduct [N where l is the length of the anode element (FIG. 2) and N isthe number of cellular compartments per unit of length. Thus, inrewritten form Equation 3 becomes =N.K g2a (4) This relationship isshown in FIG. 5 which is a graph showing the pumping speed S per unit ofanode length I vs. the cathode-anode spacing 1:, assuming the width gbetween cathode plates is kept constant. The graph shows thesubstantially linear relationship between S/l and a.

However, the intrinsic speed S cannot in general be realized since asthe anode height h increases, or in other words as the cathode anodespacing a decreases, the conductance into the pumping region iscontinuously decreased, whence the rate at which molecules can be pumpedis reduced from the rate that would exist in the absence of conductancelimitation.

The conductance C into the cellular compartments can be approximated byassuming that the conductance C through the cathode-anode space isrepresented by a slab line in which case the conductance C (for air) forthe two cathode-anode spaces is represented by the following expression:

a l C=2 200.8 (5) C a. I 401.6 b 6) wherein Z: is the width of thecellular anode 23 (FIG. 4). FIG. 6 is a graph of conductance per unit ofanode length I vs. the cathode-anode spacing.

By dividing Equation 4 by Equation 6, we obtain the followingexpression:

This expression (7) does not give the actual usable speed S or thepumping assembly at the entrance to the interaction region between thecathodes because some of the cellular compartments are distributed in apath leading away irom the entrance to the interaction region. Thus, agreater volume of gas will flow to the cellular compartments positionedadjacent the entrance than those positioned a distance from theentrance. Taking this distributed pumping relationship into account, theactual usable speed 8' of the pumping assembly is related to theintrinsic speed S of the pumping assembly by the follow- The resultingEquation 9 will be in terms of a, b, g, N and K. By measuring the speedof a single cell (where conductance is not a significant tactor) forfixed g and a and the desired parameters on which K depends, K can bedetermined from Equation 2. Then the usable speed S can be plotted vs.cathode-anode spacing a tor the given b, g, N and K from Equation 9. Thedashed line in FIG. 5 shows the actual usable speed for the particularintrinsic speed illustrated by the solid line in that figure. Asillustrated there, the usable speed reaches an optimum at a=a Since thecathode-anode spacing a is directly related to the anode height h by therelationship g=h+2a, for any particular gap width g the optimum usablepumping speed S could be selected in terms of the anode height h insteadof the cathode-anode spacing a as outlined above.

In some instances such as extremely low pressures, it may not bedesirable to operate at the optimum usable speed since the properties ofthe glow discharge may deterior ate for wide cathode anode spacings.Therefore, in selecting the cathode-anode spacing for pumps continuouslyoperating at these low pressures, the usable speed S can be plotted anda cathode-anode spacing a smaller than a, selected which does notsacrifice too much usable speed.

If it is desired to obtain the optimum cathode anode spacing tor a givengap where the parameters of magnetic field intensity B, voltage V, cellgeometry, cathode material, pumped gas, and operating pressure of thepump may change or can be varied, the procedure outlined below can befollowed. S/l can be plotted vs.. a for given b, g, and N and a numberof arbitrarily selected values of K over a broad range giving a familyof curves as shown in FIG. 7. Therefore, the above-mentioned parameterscan be varied as desired and with each particular combination anindication of the usable speed S'/l and the optimum cathode-anodespacing a can be obtained by a measurement of the intrinsic speed of asingle cell S to determine the actual value of K.

By plotting a family of curves such as those illustrated in FIG. 7 acritical range for the cathode-anode spacing a can be determined whichwill provide optimum usable pumping speed 5' over a wide range of K.FIG. 7 is an actual plot of T VS.

fora pumping assembly as described above with g: 1.25", b=3, and N =l2.As shown by the graph, if the value of a is located within the range ofapproximately 0.2" to 0.35" the usable pumping speed S will be nearlymaximum for an extremely wide range of K. A value of a midway of thisrange such as 0.275" will provide nearly maximum pumping speed S tor allvalues of K shown, and a can be adjusted up or down Within this criticalrange of 0.20 to 0.35" when the value of K during operation of the pumpis expected to be small or large, respectively. This critical range hasbeen shown to hold .true for gaps of 1.15" to 1.35. It is apparent fromthe curves in FlG. 7 that for the larger values of K it becomes more tolie within this critical range for a since the slope of the curve ofS'/l vs. a becomes greater adjacent the point in the curve for greatervalues of K.

The results of tests on structures designed by the method outlined aboveagreed closely with the computed results. An electrical vacuum pump witha gap approximately the same as the gap for which the curves in FIG. 7were drawn operated on a K level substantially the same as the curve torIQ=% and had a cathodeato-anode gap of a=0.32". According to FIG. 7 thepump was operating with a speed about of its maximum usable speed. Priorpumps of this type with a gap approximately the same, as the gap forwhich the curves in FIG. 7 were drawn operated on :a K levelsubstantially the same as the curve for K= /a and had a cathodeauode gapof a=0.12. According to FIG. 7 those pumps were operating with a speedabout 78% of maximum usable speed.

The theory used to obtain the optimum cathode-anode spacing may berefined in the following ways: the eflect of the aperture on conductancecan be taken into account; a more precise form for the slab lineconductance can be obtained graphically than is given by Equation 6; andempirically determined values of S :as a function of: h or a can beemployed instead of the simple expression of Equation 2. Theserefinements do not change the critical range of 0.20-0.35" tor a torgaps between 1.15" and 1.35".

What is claimed is:

1. A glow discharge apparatus including an anode member subdivided intoa plurality of lesser hollow openended cellular compartments, cathodemembers disposed opposite the open ends of said cellular compartments,and means for producing and directing a magnetic field coaxially of saidlesser cellular compartments tor enhancing the glow discharge current,said cathode members being spaced from one another by a value between1.15 and 1.35" and the cathode-anode spaces being within the range of0.2" to 0.35".

2. A glow discharge apparatus including an anode member subdivided intoa plurality of lesser hollow open-ended circular compartments, cathodemembers disposed opposite and equally-spaced from the open ends of saidcellular compartments, and means for producing and directing a magneticfield coaxially of said lesser cellular compartments for enhancing theglow discharge, said cathode members being spaced from one another byapproximately 1.25 and each of said cathode members being spaced fromsaid anode member by a distance of 0.2 to 0.35.

3. A glow discharge getter ion vacuum pump apparatus including a pair ofparallel spaced apart cathode plates, an anode structure disposed midwaybetween said cathode plates and having a plurality of glow dischargepassageways therein, means for applying a high voltage between saidanode structure and said cathode plates, said anode structure and saidcathode plates adapted when energized by said high voltage means toproduce a glow discharge therebetween tor pumping gaseous matter, andmeans for producing and directing a magnetic field coaxially of saidglow discharge passageways for enhancing the pumping speed of the pump,said cathode members being spaced from one another by a value between1.l5" and 1.35 and each of said cathode plates being spaced irom saidanode structure by a distance falling the range of 0.2 to 0.35".

4. A glow discharge getter ion vacuum pump apparatus including a pair ofparallel spaced apart cathode plates, an anode structure disposed midwaybetween said cathode plates and having a plurality of glow dischargepassageways therein, means for applying a high voltage falling withinthe range of 0.2 to 0.35.

5. A glow discharge getter ion vacuum pump apparatus including, a pairof parallel cathode plates spaced apart a given distance g, an anodestructure disposed midway between said cathode plates and having aplurality of glow discharge passageways therein, there being a givennumber N passageways of given width b per unit of length 1, means forapplying a high voltage between said anode structure and said cathodeplates adapted when energized by said high voltage means to produce 29 aglow discharge therebetween for pumping gaseous matter, and means forproducing and directing a magnetic field coaxially of said glowdischarge passageways for enhancing the pumping speed of said pump, eachof said cathode plates being spaced from said anode structure adeterminable distance a so as to give maximum pumping speed when tanh 7:

is a maximum where and K is a constant within the range of A to 5 litersper inch second.

No references cited.

1. A GLOW DISCHARGE APPARATUS INCLUDING AN ANODE MEMBER SUBDIVIDED INTOA PLURALITY OF LESSER HOLLOW OPENENDED CELLULAR COMPARTMENTS, CATHODEMEMBERS DISPOSED OPPOSITE THE OPEN ENDS OF SAID CELLULAR COMPARTMENTS,AND MEANS FOR PRODUCING AND DIRECTING A MAGNETIC FIELD COAXIALLY OF SAIDLESSER CELLULAR COMPARTMENTS FOR EN-